Door ajar light stays on when humidity is high.

I have a 1999 Ford F250 Super Duty crew cab 2 wd 7.3 L diesel. WHen we have a lot of fog the door ajar light starts malfunctioning turning the dome lights on and off. I don't know where to find the door switch in these newer model vehicles. I am from the old school and I can't find my shade tree! ;-)
I would appreciate any help.
Thanks,
Rich5202